Local Scour And Protection Of Offshore Three-Pile Foundation Under Constant Current Condition

In order to cope with global warming and reduce the use of fossil fuels,China has put forward the strategy of " maritime power " and the goal of " 2030 carbon peak,2060 carbon neutrality." Marine energy development projects have mushroomed,especially the construction of offshore wind farms.However,unlike land wind farms,offshore wind power pile foundations are always subject to water erosion.Under the action of water erosion,local erosion will occur around the foundation,resulting in the decrease of the buried depth of the pile foundation and the decrease of the bearing capacity.The erosion depth will seriously threaten the stability and durability of the overall structure of the wind turbine.In order to improve the stability of the pile foundation and ensure the normal operation of the unit,the multi-pile foundation form with better comprehensive bearing capacity and higher stability can be used,and certain scour protection measures can be taken to reduce the local scour depth around the pile.Based on this background,this paper carried out relevant research work.With the help of computational fluid dynamics numerical simulation technology,the local scour characteristics and scour protection measures of three-pile foundation were studied in depth.The main research contents and conclusions are as follows :(1)Firstly,the local scour mechanism of pile foundation is analyzed.Then,the existing research is sorted out in detail from four aspects : flow condition,pile foundation form,bed characteristics and scour protection,and the research object and research method are clarified.(2)Three-dimensional numerical scouring model of flow,pile foundation and sediment is established by CFD software FLOW-3D,and the numerical model is verified from the distribution of flow field around the pile and the development of scouring depth by using the test results of Roulund et al.to ensure the accuracy and feasibility of the numerical model.(3)The local scour numerical model is used to simulate the local scour of the three-pile foundation under different pile foundation layouts(pile spacing,flow angle of attack),flow conditions(flow velocity,flow depth)and bed sediment parameters(sediment median particle size).The influence of different factors on the local scour characteristics of the three-pile foundation is explored,and the maximum scour depth formula with high accuracy is used to fit the scour depth under each working condition.The results show that in terms of pile foundation layout,it is found that there are optimal pile spacing and the most unfavorable pile spacing in the three-pile foundation.And when the water attack angle is 30 °,the local scour depth of the three-pile foundation is the largest.In terms of water flow conditions,the scour depth of the three-pile foundation gradually increases with the increase of incoming flow velocity and water flow depth.In terms of bed sediment parameters,it is found that the local scour depth of the three-pile foundation does not decrease monotonously with the increase of sediment particle size.(4)The retaining ring protection is applied to the scour protection of the three-pile foundation,and the influence of the installation height and the outer diameter of the retaining ring on the local scour protection effect of the three-pile foundation under the condition of unidirectional constant current is explored.The post-processing technology was used to analyze the flow field around the pile under various working conditions,and the protection mechanism of the retaining ring on the three-pile foundation was studied.The results show that the development of local scour around the pile can be effectively inhibited by reducing the installation height of the retaining ring and expanding the outer diameter of the retaining ring.This is because the existence of the retaining ring reduces the strength of the submerged flow in front of the pile.